'\" et
.TH FREOPEN "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
freopen
\(em open a stream
.SH SYNOPSIS
.LP
.nf
#include <stdio.h>
.P
FILE *freopen(const char *restrict \fIpathname\fP, const char *restrict \fImode\fP,
    FILE *restrict \fIstream\fP);
.fi
.SH DESCRIPTION
The functionality described on this reference page is aligned with the
ISO\ C standard. Any conflict between the requirements described here and the
ISO\ C standard is unintentional. This volume of POSIX.1\(hy2017 defers to the ISO\ C standard.
.P
The
\fIfreopen\fR()
function shall first attempt to flush the stream associated with
.IR stream
as if by a call to
.IR fflush ( stream ).
Failure to flush the stream successfully shall be ignored. If
.IR pathname
is not a null pointer,
\fIfreopen\fR()
shall close any file descriptor associated with
.IR stream .
Failure to close the file descriptor successfully shall be ignored.
The error and end-of-file indicators for the stream shall be
cleared.
.P
The
\fIfreopen\fR()
function shall open the file whose pathname is the string pointed to by
.IR pathname
and associate the stream pointed to by
.IR stream
with it. The
.IR mode
argument shall be used just as in
\fIfopen\fR().
.P
The original stream shall be closed regardless of whether the
subsequent open succeeds.
.P
If
.IR pathname
is a null pointer, the
\fIfreopen\fR()
function shall attempt to change the mode of the stream to that
specified by
.IR mode ,
as if the name of the file currently associated with the stream had
been used. In this case, the file descriptor associated with the stream
need not be closed if the call to
\fIfreopen\fR()
succeeds. It is implementation-defined which changes of mode are
permitted (if any), and under what circumstances.
.P
After a successful call to the
\fIfreopen\fR()
function, the orientation of the stream shall be cleared,
the encoding rule shall be cleared,
and the associated
.BR mbstate_t
object shall be set to describe an initial conversion state.
.P
If
.IR pathname
is not a null pointer, or if
.IR pathname
is a null pointer and the specified mode change necessitates the file
descriptor associated with the stream to be closed and reopened, the
file descriptor associated with the reopened stream shall be allocated
and opened as if by a call to
\fIopen\fR()
with the following flags:
.TS
center box tab(!);
cB | cB
l | l.
\fIfreopen\fP(\^) Mode!\fIopen\fP(\^) Flags
_
\fIr\fR or \fIrb\fR!O_RDONLY
\fIw\fR or \fIwb\fR!O_WRONLY|O_CREAT|O_TRUNC
\fIa\fR or \fIab\fR!O_WRONLY|O_CREAT|O_APPEND
\fIr+\fR or \fIrb+\fR or \fIr+b\fR!O_RDWR
\fIw+\fR or \fIwb+\fR or \fIw+b\fR!O_RDWR|O_CREAT|O_TRUNC
\fIa+\fR or \fIab+\fR or \fIa+b\fR!O_RDWR|O_CREAT|O_APPEND
.TE
.SH "RETURN VALUE"
Upon successful completion,
\fIfreopen\fR()
shall return the value of
.IR stream .
Otherwise, a null pointer shall be returned,
and
.IR errno
shall be set to indicate the error.
.SH ERRORS
The
\fIfreopen\fR()
function shall fail if:
.TP
.BR EACCES
Search permission is denied on a component of the path prefix, or the
file exists and the permissions specified by
.IR mode
are denied, or the file does not exist and write permission is denied
for the parent directory of the file to be created.
.TP
.BR EBADF
The file descriptor underlying the stream is not a valid file
descriptor when
.IR pathname
is a null pointer.
.TP
.BR EINTR
A signal was caught during
\fIfreopen\fR().
.TP
.BR EISDIR
The named file is a directory and
.IR mode
requires write access.
.TP
.BR ELOOP
A loop exists in symbolic links encountered during resolution of the
.IR path
argument.
.TP
.BR EMFILE
All file descriptors available to the process are currently open.
.TP
.BR ENAMETOOLONG
.br
The length of a component of a pathname is longer than
{NAME_MAX}.
.TP
.BR ENFILE
The maximum allowable number of files is currently open in the system.
.TP
.BR ENOENT
The
.IR mode
string begins with
.BR 'r' 
and a component of
.IR pathname
does not name an existing file, or
.IR mode
begins with
.BR 'w' 
or
.BR 'a' 
and a component of the path prefix of
.IR pathname
does not name an existing file, or
.IR pathname
is an empty string.
.TP
.BR ENOENT " or " ENOTDIR
.br
The
.IR pathname
argument contains at least one non-\c
<slash>
character and ends with one or more trailing
<slash>
characters. If
.IR pathname
without the trailing
<slash>
characters would name an existing file, an
.BR [ENOENT] 
error shall not occur.
.TP
.BR ENOSPC
The directory or file system that would contain the new file cannot be
expanded, the file does not exist, and it was to be created.
.TP
.BR ENOTDIR
A component of the path prefix names an existing file that is neither
a directory nor a symbolic link to a directory, or the
.IR pathname
argument contains at least one non-\c
<slash>
character and ends with one or more trailing
<slash>
characters and the last pathname component names an existing file that
is neither a directory nor a symbolic link to a directory.
.TP
.BR ENXIO
The named file is a character special or block special file, and the
device associated with this special file does not exist.
.TP
.BR EOVERFLOW
The named file is a regular file and the size of the file cannot be
represented correctly in an object of type
.BR off_t .
.TP
.BR EROFS
The named file resides on a read-only file system and
.IR mode
requires write access.
.P
The
\fIfreopen\fR()
function may fail if:
.TP
.BR EBADF
The mode with which the file descriptor underlying the stream was
opened does not support the requested mode when
.IR pathname
is a null pointer.
.TP
.BR EINVAL
The value of the
.IR mode
argument is not valid.
.TP
.BR ELOOP
More than
{SYMLOOP_MAX}
symbolic links were encountered during resolution of the
.IR path
argument.
.TP
.BR ENAMETOOLONG
.br
The length of a pathname exceeds
{PATH_MAX},
or pathname resolution of a symbolic link produced an intermediate
result with a length that exceeds
{PATH_MAX}.
.TP
.BR ENOMEM
Insufficient storage space is available.
.TP
.BR ENXIO
A request was made of a nonexistent device, or the request was outside
the capabilities of the device.
.TP
.BR ETXTBSY
The file is a pure procedure (shared text) file that is being executed
and
.IR mode
requires write access.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Directing Standard Output to a File"
.P
The following example logs all standard output to the
.BR /tmp/logfile
file.
.sp
.RS 4
.nf

#include <stdio.h>
\&...
FILE *fp;
\&...
fp = freopen ("/tmp/logfile", "a+", stdout);
\&...
.fi
.P
.RE
.SH "APPLICATION USAGE"
The
\fIfreopen\fR()
function is typically used to attach the pre-opened
.IR streams
associated with
.IR stdin ,
.IR stdout ,
and
.IR stderr
to other files.
.P
Since implementations are not required to support any stream mode changes
when the
.IR pathname
argument is NULL, portable applications cannot rely on the use of
\fIfreopen\fR()
to change the stream mode, and use of this feature is discouraged. The
feature was originally added to the ISO\ C standard in order to facilitate changing
.IR stdin
and
.IR stdout
to binary mode. Since a
.BR 'b' 
character in the mode has no effect on POSIX systems, this use of the
feature is unnecessary in POSIX applications. However, even though the
.BR 'b' 
is ignored, a successful call to
.IR freopen \c
(NULL, "\fIwb\fR", \fIstdout\fR) does have an effect. In particular,
for regular files it truncates the file and sets the file-position
indicator for the stream to the start of the file. It is possible that
these side-effects are an unintended consequence of the way the feature
is specified in the ISO/IEC\ 9899:\|1999 standard, but unless or until the ISO\ C standard is changed,
applications which successfully call
.IR freopen \c
(NULL, "\fIwb\fR", \fIstdout\fR) will behave in unexpected ways on
conforming systems in situations such as:
.sp
.RS 4
.nf

{ appl file1; appl file2; } > file3
.fi
.P
.RE
.P
which will result in
.BR file3
containing only the output from the second invocation of
.IR appl .
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "Section 2.5" ", " "Standard I/O Streams",
.IR "\fIfclose\fR\^(\|)",
.IR "\fIfdopen\fR\^(\|)",
.IR "\fIfflush\fR\^(\|)",
.IR "\fIfmemopen\fR\^(\|)",
.IR "\fIfopen\fR\^(\|)",
.IR "\fImbsinit\fR\^(\|)",
.IR "\fIopen\fR\^(\|)",
.IR "\fIopen_memstream\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<stdio.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
